The present invention relates to coaxial cable and termination assemblies and, more particularly, to cable and termination assemblies for high voltage and high frequency applications.
A coaxial cable is a necessary and common component of the ignition systems of internal combustion engines, including gas turbine engines, used in aircraft, marine and automotive applications, as well as radar and laser power supply applications. In some applications, the cable is used to conduct high voltage and high frequency current for short durations typically 10-200 microseconds, in hostile environments which provide extreme temperatures and corrosive elements. Such voltages can be from 20,000 to 50,000 volts at frequencies exceeding 1 megahertz.
A typical coaxial cable used in such an application includes a stranded wire core, a covering of dielectric material such as a silicone or fluorosilicone a fiber glass braid covering the dielectric layer and an outer jacket of rubber or silicone. In some existing designs, the outer layers include an outer conductor of a metallic wire braid which is connected as a ground return. When the stranded central conductor transmits high frequency current, the voltage induced inside the conductor which opposes the applied voltage becomes sufficiently large to force the current to distribute itself so that the greater proportion flows near the surface of the conductor and less near the center. This phenomenon is known as "skin effect."
While negligible at low frequencies, skin effect increases with increasing frequency to such a degree that at high frequencies the major portion of the current flows near the surface. So little current flows in the interior of the conductor at these high frequencies that the utilized portion of the conductor is in the shape of a thin tube. Since such a small portion of the conductor is used the resistance per unit length of the conductor increases.
One attempt to design a coaxial cable which compensates for skin effect is disclosed in Sugi et. al. U.S. Pat. No. 3,163,836. That patent discloses high frequency electric conductors in which the wires constituting the cable are arranged in layers insulated one from another and comprise one or more elemental wires or tapes, the pitch of the wires or tapes in the respective layers being chosen so as to minimize the skin effect. The layers are stranded about a central core of plastic insulating material. While such a design may effectively transmit high frequency current, a disadvantage is that its fabrication cost is relatively high since multiple layers of conductor material are required to be stranded about a central core.
Accordingly, there is a need for a high voltage high-frequency cable which possesses increased efficiency over comparably sized prior art cable designs, yet is relatively inexpensive to manufacture.